RPM is an estimate of a player’s on-court impact on team performance, measured in net point differential across 100 offensive and defensive possessions. It takes into account a player’s teammates, the opponent, and some other factors. The formula is fairly complicated, but you can find more information about it here. ESPN updates the numbers nightly, once all the games are completed.
Nowitzki places fourth among all NBA power forwards in RPM at 3.79, behind only Golden State’s Draymond Green, Atlanta’s Paul Millsap, and Cleveland’s Kevin Love. He ranks ahead of the likes of Blake Griffin, Chris Bosh, and some of the other excellent big men in the NBA.